Method and device for starting a drive train

ABSTRACT

In a method and a drive for starting a drive train, with a drive shaft ( 2 ), a drive motor ( 4 ) connected to an electrical grid ( 12 ) and with a differential gearing ( 3 ) with three drives and outputs, wherein one output is connected to the drive shaft ( 2 ), a first drive is connected to the drive motor ( 4 ) and a second drive is connected to a differential drive ( 5 ), the drive motor ( 4 ) is started from a rotational speed of zero or approximately zero while an external braking torque acts on the drive shaft ( 2 ), and the second drive is braked in an acceleration phase of the drive shaft ( 2 ).

The invention relates to a method for starting a drive train with adrive shaft, a prime mover, and with a differential gear system withthree inputs and outputs, whereby one output is connected to the driveshaft, a first input is connected to the prime mover, and a second inputis connected to a differential drive.

The invention relates, furthermore, to an input for implementing thismethod.

BACKGROUND OF THE INVENTION

One general problem of driven machines, such as delivery systems, e.g.,pumps, compressors and fans, or such as mills, crushers, vehicles, etc.,is efficient variable-speed operation, or starting under high load,since, e.g., electrical machines, but also internal combustion engines,in most cases have a lower starting torque than their rated torque. Inaddition, electrical machines are used as the example for prime movers,but the principle applies to all possible types of prime movers, suchas, e.g., internal combustion engines.

The most frequently used electrical inputs are currently three-phasemachines, such as, e.g., asynchronous motors and synchronous motors. Inspite of high electrical power consumption, three-phase machines at restare not able to deliver this power fully mechanically; this is reflectedin high losses and a low starting torque. At the same time, the currentconsumption of a three-phase machine when starting from speed zerotypically corresponds to roughly 7 times the rated current; this causesa correspondingly high electrical load for the grid during starting.

Therefore, a three-phase machine must be designed to be correspondinglylarge so that it can deliver an input torque that corresponds to therated torque from rest and is for this reason often oversized. Also forthis reason, electrical machines are therefore often designed incombination with a frequency converter as a variable-speed input insteadof being connected directly to a grid. Thus, starting with high torquefrom speed zero can be implemented without loading the grid; theapproach is, however, expensive and associated with major losses ofefficiency. One alternative that is more cost-favorable and also betterwith respect to efficiency compared to this is the use of differentialsystems—for example according to AT 507394 A. The fundamental limitationhere, however, is that depending on the transmission ratio of thedifferential stage, only a relatively small speed range or, in theso-called differential mode, essentially no low speeds can be achievedon the drive shaft of a driven machine.

There are various possibilities for doing this. According to GermanUtility Model DE 20 2012 101 708 U, for example, the transmission ratioof the differential gear system can be fixed at 1. On this basis, thecomplete drive train can be driven with the differential drive or theprime mover can be brought to synchronous speed, and the latter cansubsequently synchronize with the grid.

The disadvantage of this approach is that the differential drive or itsfrequency converter is significantly smaller-sized than the prime moverand therefore can also only deliver a correspondingly small torque.

SUMMARY OF THE INVENTION

The object of the invention is therefore to find a solution with whichprime movers under load can either be synchronized with the grid (suchas, e.g., electrical machines coupled directly to the grid) or can beaccelerated into a speed range with high available torque (such as,e.g., in internal combustion engines), and in addition, the drivenmachine can be started up from speed zero with optimal design torque ofthe drive train.

This object is achieved with a method of the above-mentioned type insuch a way that the prime mover is started up from a speed of zero orapproximately zero, while an external braking torque acts on the driveshaft and in such a way that in an acceleration phase of the driveshaft, the second drive is braked.

This object is achieved in addition with a system as further provided asfollows.

The heart of a differential system is a differential gear system, whichin a simple embodiment can be a simple planetary gear stage with threeinputs and outputs, whereby one output is connected to the drive shaftof a driven machine, a first input is connected to the prime mover, anda second input is connected to a differential drive. Thus, the drivenmachine can be operated at variable speeds when the prime mover is atconstant speed by the differential drive compensating for the differencein speed.

In order to bring a prime mover from a standstill preferably up tosynchronous speed and in addition to start up a driven machine with hightorque from speed zero, the start-up can take place according to theinvention, e.g., as follows in 3 phases:

Phase 1: The prime mover is preferably switched to the grid using aso-called star/delta connection or alternatively (in an especiallygrid-friendly method) first using an additional system, it is brought to(at least roughly) synchronous speed and then synchronized with thegrid. In the case of an internal combustion engine, the latter is simplystarted and then accelerated. In doing so, the prime mover remainslargely free of external mechanical loads during starting, aside fromthe reaction forces of the second input of the differential gear system,which forces are caused by the mass moment of inertia and must beovercome. By implication, this means that until the prime mover hasreached its rated speed, a correspondingly small driving torque acts onthe drive shaft of the driven machine.

Phase 2: Since the full torque of the prime mover is now available, inthe second phase, the actual acceleration and starting of the drivenmachine begin under load by the second input of the differential gearstage being decelerated by means of a synchronization brake.

Phase 3: As soon as the drive shaft of the second input of thedifferential system is in the governed speed range of the differentialdrive, the latter takes over the speed control of the drive train andthe synchronization brake is released.

Preferred embodiments of the invention are the subject of the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, preferred embodiments of the invention are explained withreference to the attached drawings. Here:

FIG. 1 shows the principle of a differential system according to theinvention for one input of a pump,

FIG. 2 shows another embodiment according to the invention of adifferential system,

FIG. 3 shows another embodiment according to the invention of adifferential system with a gear driving stage,

FIG. 4 shows the speed parameters and power parameters of a differentialsystem of a pump,

FIG. 5 shows another embodiment according to the invention of adifferential system with a simplified differential drive,

FIG. 6 shows the speed parameters and power parameters following fromFIG. 5,

FIG. 7 shows another embodiment according to the invention of adifferential system with a gear shifting stage,

FIG. 8 shows the speed parameters and power parameters following fromFIG. 7,

FIG. 9 shows another embodiment according to the invention of adifferential system with a reduced speed range,

FIG. 10 shows the speed parameters and power parameters following fromFIG. 9,

FIG. 11 shows the possible speed parameters and power parametersresulting from FIG. 9 for a so-called pump turbine,

FIG. 12 shows another embodiment according to the invention of adifferential system for an internal combustion engine as a prime mover,and

FIG. 13 shows a control system for damping drive train vibrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the principle of a differential system for a drive train inthe example of a pump. Here, the driven machine 1 is the rotor of apump, which is driven via a drive shaft 2 and a differential gear system3 from a prime mover 4. The prime mover 4 is preferably a medium-voltagethree-phase machine that is connected to a grid 12 that in theillustrated example is a medium-voltage grid based on a medium-voltagethree-phase machine. The chosen voltage level depends, however, on theapplication and mainly the power level of the prime mover 4 and can haveany desired voltage level without influencing the basic function of thesystem according to the invention. According to the number of pole pairsof the prime mover 4, a design-specific operating speed range isproduced. Here, the operating speed range is that speed range in whichthe prime mover 4 can deliver a defined or desired or necessary torqueand in the case of an electrical prime mover can be synchronized withthe grid 12. A planetary carrier 7 is connected to the drive shaft 2,the prime mover 4 is connected to a ring gear 8, and a sun wheel 9 ofthe differential gear system 3 is connected to a differential drive 5.The heart of the differential system in this embodiment is thus a simpleplanetary gear stage with three inputs and outputs, whereby one outputis connected to the drive shaft 2 of the driven machine 1, a first inputis connected to the prime mover 4, and a second input is connected tothe differential drive 5.

In order to be able to optimally adapt the speed range of thedifferential drive 5, an adjusting gear system 10 is implemented betweenthe sun wheel 9 and the differential drive 5. Alternatively to theillustrated spur wheel stage, the adjusting gear system 10 can, forexample, also be made multi-stage or as a toothed belt or chain driveand/or can be combined with a planetary gearing stage. Moreover, withthe adjusting gear system 10, an axial offset for the differential drive5 can be implemented that due to the coaxial arrangement of the drivenmachine 1 and the prime mover 4 makes possible a simple design of thedifferential drive 5. A motor brake 13 that if necessary brakes thedifferential drive 5 is connected to the differential drive 5.Electrically, the differential drive 5 is linked to the grid 12 by meansof a preferably low-voltage frequency converter, consisting of amotor-side inverter 6 a and a grid-side inverter 6 b, and a transformer11. The transformer equalizes any existing voltage differences betweenthe grid 12 and the grid-side inverter 6 b, and can be omitted atvoltage equality between the prime mover 4, the grid-side inverter 6 b,and the grid 12. The inverters 6 a and 6 b are connected by a d.c.intermediate circuit and if necessary can be separated locally, wherebypreferably the motor-side inverter 6 a is positioned as near as possibleat the differential drive 5. The major advantage of this concept is thatthe prime mover 4 can be linked directly, i.e., without complex powerelectronics, to a grid 12. The equalization between variable rotor speedand fixed speed of the grid-linked prime mover 4 is implemented by thevariable-speed differential drive 5.

The torque equation for the differential system is:Torque_(differential drive)=Torque_(drive shaft) *y/x,whereby the size factor y/x is a measure of the transmission ratios inthe differential gear system 3 and in the adjusting gear system 10. Thepower of the differential drive 5 is essentially proportional to theproduct of the percentage deviation of the pump speed from its basespeed×drive shaft output. Accordingly, a large speed range in principlerequires a correspondingly large dimensioning of the differential drive5. This is also the reason why differential systems are especially wellsuited for small speed ranges, but in principle any speed range isachievable.

A differential drive 5 for a pump as a driven machine 1 has, forexample, a power of roughly 15% of the total system power. This in turnmeans that low speeds on the driven machine 1 cannot be implemented withthe differential system. If the driven machine 1 must be brought fromspeed zero with high torque into its operating speed range (this is thespeed range in which the driven machine 1 essentially operates), thiscan only be implemented by the differential drive 5 being braked (eitherelectrically or by means of a motor brake 13) and the prime mover 4being switched to the grid. The prime mover 4 in turn can only apply therated torque with difficulty from rest or it draws up to 7 times therated current in order to accelerate roughly to synchronous speed. Byusing a so-called star/delta connection, the starting current can bereduced, but thus also the attainable starting torque is reduced.

An improvement according to the invention is achieved, e.g., by thedifferential drive 5 at the beginning of starting being brought to itsmaximum possible operating speed. Due to external loads, in themeantime, the driven machine 1 remains in a range of low speed. In thisway, the prime mover 4 is brought to a speed that is necessarilyestablished depending on the speed of the driven machine 1, on the onehand, and the transmission ratio of the differential gear system 3 andan optionally present adjusting gear system 10, on the other hand. Then,the differential drive 5 is regulated such that its speed remains withinits governed speed range, while the prime mover 4 is switched to thegrid 12 with or without a so-called star/delta connection. The speedregulation or braking of the differential drive 5 in this case iscarried out preferably electrically by the inverter 6 a, 6 b or by meansof a motor brake 13.

The motor brake 13 can also be used to protect the differential drive 5from overspeeds, when, e.g., the prime mover 4 fails and the drivenmachine 1 stops or turns in the opposite direction.

FIG. 2 shows another embodiment according to the invention of adifferential system. Here, the illustrated drive train as in FIG. 1 alsohas a driven machine 1, a drive shaft 2, a differential gear system 3, aprime mover 4, and a differential drive 5 that is connected to the grid12 by means of a frequency converter 6 (consisting of motor-side andgrid-side inverters—here shown simplified as a unit) and a transformer11. Here, the differential drive 5 is also linked to the differentialgear system 3 by means of an adjusting gear system 10. In addition,however, a clutch 15 is implemented between the adjusting gear system 10and the differential gear system 3.

A synchronization brake 14 acts on the sun wheel 9 and thus on theentire drive train. When starting, in this embodiment of the inventionin a first step, the differential drive 5 and the adjusting gear system10 are decoupled by the clutch 15 from the remainder of the drive train.If the prime mover 4 is now accelerated and connected to the grid, thesun wheel 9 turns freely at the same time, and no noteworthy torque canbuild up in the entire drive train. Thus, in this case, the drivenmachine 1 also remains in a range of low speed, and the prime mover 4can be synchronized with the grid 12 without noteworthy externalcounter-torque.

In order to avoid the above-described effect of the high startingcurrent when the prime mover 4 is being synchronized, either astar/delta connection can be implemented or the prime mover 4 can bebrought to (approximately) synchronous speed by an auxiliarysystem—e.g., a small variable-speed input—and then can be synchronizedwith the grid 12. Alternatively, with the clutch 15 engaged—as alreadydescribed in FIG. 1—the prime mover 4 can be brought to speed with thedifferential drive 5. In doing so, the prime mover 4 cannot beaccelerated to its synchronous speed, but at least the starting currentthat is being established is smaller. The clutch 15 is then disengagedagain.

An alternative method for smooth grid synchronization of the electricalmachine 4 would in this case be to separate, on the one hand, thefrequency converter 6 from the differential drive 5 and, on the otherhand, the electrical machine 4 from the grid 12. Subsequently, theelectrical machine 4 can be synchronized with the grid 12 by means ofthe frequency converter 6, then the electrical machine 4 connecting tothe grid 12, and after that the frequency converter 6 can be connected(again) to the differential drive 5. Thus, the electrical machine 4 canbe switched smoothly to the grid 12. In this case, the differentialdrive 5 would begin with the variable-speed regulation of the drivetrain only as soon as the drive shaft of the differential gear system 3connected to the sun wheel 9 lies in the governed speed range of thedifferential drive 5.

As soon as the prime mover 4 accelerates above a certain speed and thedriven machine 1 in the meantime is turning only slowly, on the sunwheel 9, a speed is established that is high according to thetransmission ratio of the differential gear system 3 and that (withconsideration of the adjusting gear system 10) is above the allowedgoverned speed range for the differential drive 5. The governed speedrange is the speed range in which the differential drive 5 works inorder to be able to implement the operating speed range of the drivenmachine 1. The governed speed range is determined in doing so mainly bythe voltage limits, current limits and speed limits that have beenspecified by the manufacturer. In this phase, the differential drive 5cannot be connected to the grid 12. In another step, therefore, thesecond input of the differential gear system 3, which input is connectedto the sun wheel 9, is decelerated with the synchronization brake 14 toa speed that is in the governed speed range of the differential drive 5.This can, depending on the braking system produced or the requirementsof the drive train, be done both with and without regulation in terms ofspeed/torque. Subsequently, the differential-drive-side part of theclutch 15 is preferably synchronized (preferably by means of thedifferential drive 5) with the speed of the second input of thedifferential gear system 3, and then the clutch 15 is engaged. Theclutch 15 is preferably a positive jaw clutch or a non-positivemulti-disk clutch. One advantage of the non-positive multi-disk clutchis that if it is designed for this purpose, synchronization of the twoclutch halves is not necessary. The clutch 15 can be omitted when thedifferential drive 5 is designed for the speeds established during thestart-up process. Thus, the motor brake 13 can subsequently replace thesynchronization brake 14.

In order to be able to achieve high torque, which lies above the torqueof the differential drive 5, the synchronization brake 14 or the servicebrake 13 can also be provided to increase the torque in the drive trainin the dynamic (normal) operation of the differential system—i.e., herethe differential drive 5 and the synchronization brake 14 or the servicebrake 13 act in the same torque direction, in which case acorrespondingly high overall torque in the drive train can be achieved.

By actuating the synchronization brake 14, the drive shaft 2 isnecessarily accelerated, whereby the torque that is available for thispurpose is determined by the minimum from the braking force of thesynchronization brake 14 acting on the drive shaft 2, on the one hand,and the breakdown torque of the prime mover 4, on the other hand. Thatis to say, in contrast to the starting options according to the state ofthe art, here, the multiple rated torque can be implemented as thestarting torque from speed zero since the typical breakdown torque of athree-phase machine is roughly 2 to 3 times its rated torque. Inprinciple, this starting method can also be used in, e.g., internalcombustion engines, which is sometimes necessary since in the partialspeed range, the latter can only generate a torque that is much smallerthan their rated torque.

As a synchronization brake 14, for example, a disk brake (=mechanicalbrake) is used, with which the latter can also serve as a service andsafety brake for the differential drive 5. Thus, the synchronizationbrake 14 can in principle also perform the function of the motor brake13 that is shown in FIG. 1.

Alternatively, however, any type of brake can be used. In particular,so-called retarders are suggested here. First of all, the group ofhydrodynamic retarders (=hydraulic brake) should be named here. For themost part, hydrodynamic retarders work with oil or water that ifnecessary is routed into a converter housing. The converter housingconsists of two rotationally-symmetrical blade wheels that are oppositeone another, and prior to this, a rotor that is connected to the drivetrain of the unit, and a stationary stator. The rotor accelerates thesupplied oil, and the centrifugal force presses it to the outside. Theshape of the rotor blades routes the oil into the stator that in thisway induces a braking torque in the rotor and subsequently then alsobrakes the entire drive train. In an electrodynamic retarder(=electrical brake), e.g., an eddy-current brake, e.g., two steel disks(rotors) that are not magnetized are connected to the drive train. Inbetween is the stator with electrical coils. When current is applied byactivation of the retarder, magnetic fields are generated that areclosed by the rotors. The magnetic fields in opposite directions thengenerate a braking action. The heat that is produced is released againby, e.g., internally-ventilated rotor disks.

An important advantage of a retarder as a service brake is its freedomfrom wear and tear and ease of control.

The system according to the invention can also be used to operate theprime mover 4 in phase-shifting operation. That is to say, the primemover 4 can deliver or draw reactive current into or out of the grid 12without the driven machine 1 being operated. This applies in particularto power plants. In this case, the prime mover 4 is only connected tothe grid 12, without implementing the additional steps of the describedstarting process. This is done only when the driven machine 1 has tointegrate the operation.

FIG. 3 shows another embodiment according to the invention of adifferential system with a gear driving stage 16. This gear drivingstage 16 makes it possible to match the speed range for the drive shaft2 or for the driven machine 1 according to the transmission ratio of thegear driving stage 16. The use of a gear driving stage 16 is necessaryand advantageous when the speed level that results based on thetechnical parameters of, e.g., an economical prime mover 4 and of anefficient differential system does not correspond to the requiredoperating speed range of a driven machine 1. A resulting advantage isthat if the gear driving stage 16 as shown is a spur wheel stage, thedifferential drive 5 can be positioned without an adjusting gear system10 according to FIGS. 1 and 2 coaxially to the prime mover 4 on the sideof the differential gear system 3 facing away from the prime mover.

In order to achieve a higher transmission ratio in the differential gearsystem 3 that is necessary due to the elimination of the adjusting gearsystem 10, it is suggested to use so-called stepped planets instead ofsimple planets. These stepped planets each consist of two gear wheelsthat are connected in a torque-proof manner with a different diameterand preferably different toothing geometry. The ring gear 8 then engagesthe smaller-diameter gear wheel of the stepped planet, and the sun wheel9 engages the second gear wheel of the stepped planet. As analternative, however, instead of the spur wheel stage depicted in FIG.1, a planetary gearing stage can now also be produced as an adjustinggear system 10. Both the synchronization brake 14 and the clutch 15can—depending on the desired speed/torque ratios—be positioned either infront of or behind the adjusting stage 10.

The connecting shaft 26 between the differential gear system 3 and thedifferential drive 5 is preferably an electrically nonconductive fibercomposite shaft. If the connecting shaft 26 is an electricallyconductive shaft, preferably an insulating element can then be installedbetween the differential gear system 3 (or, if present, the adjustinggear system 10) and the differential drive 5 in order to keep unwantedelectrical current away from the differential gear system 3.

Thus, the differential system consists of a number of components that isas small as possible and, moreover, has an optimum overall efficiency.The motor brake 13 in the illustrated configuration also performs thefunction of the synchronization brake 14 from FIG. 2. The disadvantageof this embodiment compared to the one according to FIG. 2 is that thedifferential drive 5 must be designed for the starting process accordingto the invention for a higher speed, whereby the differential drive 5 atspeeds above the governed speed range is preferably separated from thegrid. Thus, speeds outside of the governed speed range need be onlymechanically tolerated. In addition, to make matters worse, thetransmission ratio of the differential gear system 3 must be higher thanfor the design according to FIG. 2, since here the adjusting gear system10 is absent. In principle, however, the additional use of an adjustinggear system 10 is also possible for the variant according to FIG. 3, asa result of which the transmission ratio of the differential gear system3 can be smaller. Moreover, a clutch 15 and a synchronization brake 14can also be implemented between the second input of the differentialgear system 3 or sun wheel 9 and the differential drive 5.

In principle, this embodiment can also be used for power plants,especially wind power plants and hydro-electric power stations, as thedriven machine 1. In this case, compared to, e.g., a pump as the drivenmachine 1, the power flow direction is reversed, and the prime mover 4works as a generator. If necessary, there can be one or more furthergear stages between the gear driving stage 16 and the driven machine 1,which gear stages are then preferably designed as planetary gear stages.

Another advantage of this embodiment with gear driving stage 16 is thata coaxial hollow shaft 27 to the driven machine 1 can be easilyimplemented. By means of this hollow shaft 27, the turning drivenmachine 1 can be easily supplied electrically or hydraulically. Here,preferably rotational transmission 28 to the side of the gear drivingstage facing away from the driven machine is applied. In principle, amechanical rod can also be routed in the bushing 27 and thus bytranslational or rotary motion, e.g., the blades of a pump rotor can bemechanically adjusted.

If the differential system and the gear driving stage 16 are provided asso-called “stand-alone” variants, the drive shaft 2 and the prime mover4 are preferably connected by means of a clutch 17, 18.

FIG. 4 shows the parameters of speed and power of a differential system,for example for a pump. The illustration shows power and speed valuesfor a pump as a driven machine 1, a prime mover 4, and a differentialdrive 5, in each case plotted over the speed values of the drive shaft 2(“pump speed”). The prime mover 4 is connected to the grid 12 and thusits speed (“motor speed”) is constant—in the illustrated example,roughly 1,500 μminute for a four-pole three-phase machine in a 50-Hzgrid. The operating speed range for the drive shaft 2 ranges from 68% to100%, whereby 100% is the chosen rated or maximum working point.According to the transmission ratio of the differential system, thespeed of the differential drive 5 (“servo speed”) ranges from −2,000l/min to 1,500 μmin. This means that the differential drive 5 isoperated by generator (−) and motor (+). Since the maximum requiredpower of the differential drive 5 in the generator (−) region (roughly110 kW) is smaller than that in the motor (+) region (roughly 160 kW),the differential drive 5 in the generator (−) region can be operated inthe so-called field weakening range, with which for the differentialdrive 5, a higher speed—but with reduced torque—can be implemented.Thus, the speed range for the driven machine 1 can be easily expanded.

Another possibility for expanding the speed range for the driven machine1 features the so-called 87-Hz characteristic for the operation of thefrequency converter 6. The principle here is the following: motors cantypically be operated in star (400 V) or delta (230 V). If a motor isoperated as usual with 400 V in a star connection, then the ratedworking point is reached at 50 Hz. This characteristic is set in thefrequency converter. A motor can also be operated with 400 V in a deltaconnection, however, and the frequency converter can be parameterizedsuch that it reaches 50 Hz at 230 V. In this way, the frequencyconverter reaches its rated voltage (400 V) only at 87 Hz (√3×50 Hz).Since the motor torque is constant up to the rated working point, ahigher power is achieved with the 87-Hz characteristic. Here, however,it is to be considered that compared to the star connection, in thedelta connection, the current is higher by √3. That is to say, thefrequency converter must be sized to be stronger. Moreover, in themotor, due to the higher frequency, even higher losses arise for whichthe motor must be thermally designed. Ultimately, however, with the87-Hz characteristic, a correspondingly (√3) higher speed range isattained with—in contrast to field weakening—a torque that is notreduced.

The “T” point in FIG. 4 marks the so-called “base speed” of the driveshaft 2, at which the speed of the differential drive 5 is equal tozero. Ideally, this “T” point is placed in a working range in which theunit is operated over large time intervals. At this operating point, themotor brake 13 can be activated, with which the differential drive 5need not be operated, and subsequently, associated losses and wear andtear are avoided. In the motor (+) region of the characteristic diagram,the input is driven in parallel from the prime mover 4 and thedifferential drive 5. The sum of the two powers is the input power forthe drive shaft 2 (“system power”)—minus the system losses that arise.In the generator (−) region, the prime mover 4 must compensate for thepower of the differential drive 5 (“servo power”), as a result of whichthe overall system power (“system power”) is the input power of theprime mover 4 (“motor power”) minus the power of the differential drive5. That is to say, in terms of efficiency, the motor (+) region isbetter. This matches extremely well the illustrated exemplary frequencydistribution (“probability”) of the load distribution in continuousoperation of the unit, which shows a large part of the duration ofoperation in the motor (+) region. As dictated by operation, however, anoperation at lower pump speeds is also necessary, whereby here theproportional dwell time decreases greatly with decreasing pump speed.

In principle, it can be established that the closer the pump speed(“pump speed”) is to the base speed “T,” the smaller the power flow viathe differential drive 5, and thus the overall system efficiency is alsovery high. Since, with increasing pump speed, the required input poweralso rises, however, compared to an input according to the state of theart, the required size of the prime mover 4 can be reduced by the sizeof the differential drive 5 by the parallel input of the prime mover 4and of the differential drive 5.

As was already mentioned initially, according to German Utility Model DE20 2012 101 708 U, the transmission ratio of the differential drive canbe fixed at 1 using a differential blocking device. Thus, it is possiblewith the differential drive 5 to accelerate the complete drive train tothe synchronous speed of the prime mover 4 and then to synchronize thelatter with the grid. Subsequently, the differential drive 5 can bealternately switched off, and the prime mover 4 drives the drivenmachine 1 with synchronous speed alone. In addition, the differentialdrive 5 can drive the driven machine 1 parallel to the prime mover 4,with which a higher overall drive train power can be implemented. Thus,two steady-state operating points of the drive train can be implementedwith the differential blocking device and the motor brake 13. In oneespecially economical embodiment, the differential drive is madelower-power such that with it, only the prime mover 4 is synchronizedwith the grid 12, or the differential blocking device. This canalternatively also be accomplished, however, by optional driving of theoutput or of the first input of the differential gear system 3.

If the prime mover 4 is to be synchronized only smoothly with the grid,the latter can be synchronized to the grid with a small frequencyconverter. Then, the second input is braked to speed zero by means ofthe synchronization brake 14 and thus the driven machine is accelerated.Since no differential drive 5 is provided in this simple embodiment,however, only a fixed operating speed can thus be achieved.

FIG. 5 shows another embodiment according to the invention of adifferential system with a simplified differential drive. In thisvariant embodiment, the grid-side inverter 6 b is replaced by a simplerectifier 19. For the most part, the latter has a higher efficiency thanan inverter 6 b and is also much more durable and economical. The solelimitation by the use of a rectifier 19 is that the differential drive 5can only continue to be operated by motor (+).

If, in the reverse case, the differential system is operated only bygenerator (−), the motor-side inverter 6 a can be replaced by arectifier 19 while maintaining the grid-side inverter 6 b.

FIG. 6 shows the parameters of speed and power following from FIG. 5 atthe same operating speed range for the drive shaft 2 as in FIG. 4(68%-100%). Due to the fact that the differential drive 5 continues onlyto be operated in the motor (+) region, the maximum power flow via thedifferential drive 5 is much greater than in the example shown above. Atthe rated working point, the required power of the differential drive 5(“servo power”) reaches roughly 500 kW, which is 50% of the total drivepower (“system power”). This results in that the frequency converter 6a, 19 must also be dimensioned to be correspondingly large. Theadvantage of this variant is that the transmission ratio of thedifferential gear system 3 can be much smaller than for the variantaccording to FIG. 3, and thus when the system starts up according to theinvention, the speed of the differential drive 5 that is the maximumthat can be attained in doing so is smaller.

FIG. 7 shows another embodiment according to the invention of adifferential system with a gear shifting stage. In the illustratedembodiment, the gear driving stage 16 is expanded by another geardriving stage 20 with a transmission ratio that is different from thegear driving stage 16. By means of a shifting device 21, it is possibleto choose between the two gear driving stages, and thus an adjustablegear system 16, 20, 21 is obtained that can implement two speed rangesfor the drive shaft 2. Alternatively, multiple shifting stages can alsobe implemented.

FIG. 8 shows the parameters of speed and power that follow from FIG. 7.In principle, the illustration contains two characteristic diagrams—eachof which is similar to FIG. 6, but each with a smaller operating speedrange for the driven machine 1. These characteristic diagrams are offsetto one another by the two-stage adjustable gear system 16, 20, 21 withwhich at the same overall operating speed range for the pump (“pumpspeed” 68%-100%), a size for the differential drive 5 that is smaller incomparison to FIG. 6 is necessary. Moreover, in the characteristicdiagram with lower system power, the differential drive 5 can beoperated in the field weakening range, since here, the torque that isnecessary for the differential system is in principle lower than itsrated torque. Thus, the operating speed range in the characteristicdiagram with the lower system power is larger than that for the secondcharacteristic diagram. The two characteristic diagrams overlap oneanother preferably in the hysteresis region “H” in order to avoidfrequent shifting between the characteristic diagrams. The hysteresisregion “H,” however, burdens a differential system that is still smallerin terms of power, and can, if no overlapping of the two characteristicdiagrams is necessary, also be smaller or be omitted entirely.

FIG. 9 shows another embodiment according to the invention of adifferential system with a reduced speed range. In principle, the drivetrain is built the same as already shown in FIG. 5. In the power system29 of the driven machine 1 (e.g., a pump, a compressor, or a fan), achoke 22 is integrated following the latter. Thus, the amount deliveredby the driven machine 1 can be choked, without for this purpose reducingthe speed of the driven machine 1. This choke 22 is conventionally usedin drives that are not variable-speed drives in order toregulate/control the delivered amount. The choke 22 can have the mostvaried embodiments, whereby a simple flap constitutes a conventionalvariant.

In principle, for the variant according to FIG. 9, the additional use ofan adjusting gear system 10 is also possible. Moreover, a clutch 15 anda synchronization brake 14 can also be implemented between the secondinput or the sun wheel 9 and the differential drive 5. Furthermore, thegear driving stage 16 is also not critically necessary.

In order to make the size of the differential drive 5 and of thefrequency converter 6 a, 19 as small as possible, instead of therectifier 19, a grid inverter 6 b can also be used, and thus the systemcan be operated by motor (+) and generator (−), as a result of which thesize of the differential drive 5 is decisively reduced. Thus, the basespeed (“T” point) moves into the middle of the operating speed range, inwhich the differential drive 5 can be braked, and thus the differentialsystem can be operated especially efficiently. Small variations of theamounts delivered (such as, for example, in pumps) or variations thatare necessary based on operation can be compensated/regulated here withthe choke 22.

One possibility for expanding the operating speed range for the drivenmachine 1 is offered, as already described in FIG. 4, by the fieldweakening range or the so-called 87-Hz characteristic for the operationof the differential drive 5 and of the frequency converter 6 a, 6 b or19.

FIG. 10 shows the parameters of speed and power that follow from FIG. 9.The chosen operating range of the differential system thus moves into aregion with a high operating frequency distribution (“probability”). Assoon as the differential drive 5 reaches the base speed (“T” point) asthe pump speed decreases, the latter is preferably braked or stopped. Alower delivery rate that is necessary for reasons of operatingtechnology is implemented by activation (regulation/control) of thechoke 22. The speeds of the differential system remain essentiallyconstant here.

FIG. 11 shows the possible parameters of speed and power for a so-calledpump turbine that follow from FIG. 9 (a choke 22 can be omitted here).In this application, the system is preferably operated by motor (+)above the base speed (“T” point) and by generator (−) below the basespeed. Here, in generator operation, the prime mover 4 works as agenerator that is connected to the grid 12. The differential drive (5)continues to be operated by motor (+) at a driven machine speed belowthe base speed due to the power flow reversal. This yields anelectrically simple system that can be implemented without a grid-sideinverter. Since, however, the power flows from the generator (4) and thedifferential drive (5) are opposite below the base speed and thus thesystem efficiency is poorer than in purely motor operation—if possiblein terms of operating technology—the procedure can be performed at afixed speed, i.e., preferably with the differential drive 5 stopped,entirely or partially in this mode of operation. Ideally, the operatingpoints are then placed such that the pump turbine at the base speed(“T”) has an optimum efficiency for the turbine operating mode.

FIG. 12 shows another embodiment according to the invention of adifferential system for an internal combustion engine 23 as the primemover. Since the internal combustion engine 23 is not connected to anelectrical grid, the required energy for the differential drive 5 istaken from the first input of the differential gear system 3 or issupplied to it. Here, two motor-side inverters 6 a are connected bymeans of a d.c. intermediate circuit and drive another differentialdrive 25. The latter is connected to the first input of the differentialgear system 3 by means of the adjusting gear system 24. The adjustinggear system 24 that is shown as single-stage can also be multi-stage ifnecessary. Thus, the energy circuit is closed, and the system can beoperated by both generator (−) and also motor (+), more or lessindependently of the grid. If the design speeds of the internalcombustion engine 23 and of the differential drive 25 go well together,the adjusting gear system 24 can be omitted, and the differential drive25 is directly (by means of a clutch) coupled to the internal combustionengine 23.

Ideally, the electrical part of the differential system, consisting ofthe differential drives 5 and 25 and the two inverters 6 a, is alsoconnected to a grid. Thus, for example, the starting scenarios that aredescribed for FIGS. 1 to 3 can be easily implemented and/or (as isconventional, e.g., in ship propulsions) a power grid can be supplied.Moreover, the integration of a shifting stage according to FIG. 7 isalso possible.

Instead of the differential drives 5 and 25 and the two inverters 6 a, ahydrostatic actuating gear can also be used. In doing so, thedifferential drives 5 and 25 are replaced by a hydrostatic pump/motorcombination, which is connected to a pressure line and which both canpreferably be adjusted in flow volume. Thus, as in the case of avariable-speed electrical differential drive, the speeds can beregulated. This also applies for applications with an electrical machineas the prime mover (4).

The major advantages that arise for the operation of an internalcombustion engine 23 in combination with a differential system are, onthe one hand, the attainable high starting torque according to theinvention and the fact that the internal combustion engine can be run inan efficiency-optimum range as soon as the differential system takesover the speed matching for the driven machine 1. Because, in contrastto a grid-coupled three-phase machine, an internal combustion engine canbe operated at variable speed, the range of possibilities forexpanding/varying the characteristic diagrams of the system is wide.

FIG. 13 shows a regulating system for damping drive train vibrations.The torque on the differential drive 5 is proportional to the torque inthe entire drive train, as a result of which torque regulation/controland also drive train damping by the differential drive 5 becomepossible. Drive train damping is defined here as the dedicatedcorrection of mostly rotational drive train vibrations (driven machine1, drive shaft 2, differential gear system 3, prime mover 4, anddifferential drive 5), which can occur transiently or constantly andwhich lead to unwanted loads in the entire drive train or in parts ofit. This is achieved by modulation of the torque and/or of the speed ofthe differential drive 5 with vibrations of the same frequency.

Such unwanted drive train vibrations or transient drive train loads canarise either by loads acting from the outside on the driven machine 1,in the drive shaft 2, the differential gear system 3, and thedifferential drive 5 themselves or by the prime mover 4 and aretypically apparent in the speed behavior or torque behavior of the drivetrain.

Preferably, the latter can be detected by measurements of speed and/orvibration in the drive train or by current measurements on the primemover 4 and/or on the differential drive 5. Direct detection of torqueis likewise possible, but for the most part is difficult to accomplish.The type of detection, however, always depends ultimately on at whichlocation in the drive train the damping is to take place and whethercouplings can be used.

If drive train vibrations are caused by, e.g., a typical operatingbehavior on the driven machine 1, and if they are to be compensated intheir action on the prime mover 4, they can be reduced or extinguishedby impressing torque vibrations in phase opposition on the differentialdrive 5. This is the case, e.g., in compressors in which design-specificvibration excitations that correlate strongly with the piston positionoccur when the piston rod is revolving. Since the respective vibrationexcitation always occurs at the same piston position, it is sufficientto know the peripheral position or rotary position, e.g., bymeasurement, in order to be able to compensate for this. The knowledgeof this vibration excitation allows selective compensation of individualor multiple vibrations at the same time. This is preferably achieved bydetecting the position of the piston rod or by one of the above-citedmethods. The necessary synchronous torque/speed matching in phaseopposition is implemented by conventional methods of signal processingpreferably with oscillators and notch-filter algorithms that simulateand evaluate the measured vibration excitation with the correctfrequencies. Incorporated into an oppositely-coupled system, thenecessary amplitudes and phase angles are thus automatically establishedfor the vibrations that have been generated for compensation and withwhich then the actuator on the differential drive 5 is activated.

As is shown by way of example in FIG. 13, a comparator circuit 30 issupplied with a target constant speed n₄ of the prime mover, on the onehand, and the speed n₂ of the drive shaft 2. A regulating system 31controls the differential drive 5 using the desired speed n_(5desired)that has been determined therefrom and the actual speed n₅ of the inputshaft of the differential drive 5 via the frequency converter 6 suchthat vibrations of the prime mover 4 are damped as well as possible oras desired. The drive train damping that is described with reference toFIG. 13 can also be used independently of all other embodiments thatwere described above.

The invention claimed is:
 1. A method for starting a drive train with adrive shaft (2), a prime mover (4), and a differential gear system (3)with three input/output components, whereby a first component of thethree input/output components is connected to the prime mover (4), asecond component of the three input/output components is connected to adifferential drive (5), and a third component of the three input/outputcomponents is connected to the drive shaft (2), comprising: starting upthe prime mover (4) while an external braking torque acts on the driveshaft (2) by way of an operational load upon the drive shaft; and assoon as the prime mover (4) has reached an operating speed, braking thesecond component in an acceleration phase of the drive shaft (2),whereby the second component is braked until the second componentreaches a speed by which a speed of the differential drive (5) lies in agoverned speed range.
 2. The method according to claim 1, wherein thesecond component of the differential drive (5) is first decoupled andafter reaching a speed at which the speed of the differential drive (5)lies in its governed speed range, the second component is coupled to thedifferential drive (5).
 3. The method according to claim 1, wherein thesecond component is braked by any of a mechanical, electrical orhydraulic brake (13, 14), connected to the differential drive (5). 4.The method according to claim 1, wherein the second component isdirectly braked mechanically, electrically, or hydraulically.
 5. Themethod according to claim 1, wherein the prime mover (4) is anelectrical machine that is connected to a power grid (12), and whereinthe electrical machine is brought to synchronous speed in connection apower grid (12) by means of a star/delta circuit, and then issynchronized with the power grid (12).
 6. The method according to claim1, wherein the prime mover (4) is accelerated with the differentialdrive (5).
 7. The method according to claim 6, wherein the prime mover(4) is an electrical machine that is connected to a power grid (12) andis connected to the power grid (12) after being accelerated by thedifferential drive (5).
 8. The method according to claim 1, wherein theprime mover (4) is an electrical machine that is connected to a powergrid (12), and wherein the electrical machine accelerates by means of afrequency converter (6) of the differential drive (5) to be synchronizedwith the power grid (12), and the electrical machine (4) is thenconnected to the power grid (12), and the prime mover (4) is thenseparated from the frequency converter (6) and the frequency converter(6) is connected again to the differential drive (5).
 9. A system forstarting a drive train with a drive shaft (2), comprising: a drive shaft(2), a prime mover (4), a differential gear system (3) with threeinput/output components, and a differential drive (5), wherein a firstcomponent of the three input/output components is connected to the primemover (4), a second component of the three input/output components isconnected to the differential drive (5), and a third component of thethree input/output components is connected to the drive shaft (2),wherein the second component is also connected to a brake (13), andwherein the brake is configured such that, after startup of the primemover and the prime mover reaching an operating speed, the brakeoperates to brake the second component in an acceleration phase of thedrive shaft until the second component reaches a speed by which a speedof the differential drive (5) lies in a governed speed range.
 10. Thesystem according to claim 9, wherein the prime mover (4) is athree-phase machine that is connected to a power grid (12).
 11. Thesystem according to claim 9, wherein the prime mover is an internalcombustion engine.
 12. The system according to claim 9, wherein thedifferential drive (5) is a three-phase machine.
 13. The systemaccording to claim 9, wherein the differential drive is a hydraulicpump/motor.
 14. The system according to claim 9, wherein thedifferential drive (5) is connected to the second component via anadjusting gear system stage (10).
 15. The system according to claim 9,wherein the brake (13) is a disk brake.
 16. The system according toclaim 9, wherein the brake (13) is a retarder.
 17. The system accordingto claim 9, wherein the differential drive (5) is connected to thesecond component via a clutch (15).
 18. The system according to claim 9,wherein the differential drive (5) and brake act in a same torquedirection.